What I don't understand is why are Bitcoin sites keeping such amounts in their hot wallets in the first place? Bank robberies happen, that's why traditional ban

What I don't understand is why are Bitcoin sites keeping such amounts in their hot wallets in the first place? Bank robberies happen, that's why traditional banks don't keep a huge amount of cash in the teller's office. Why not just transfer almost everything into cold wallets immediately? This way, the worst that could happen is every once in a while a human needs to bring a cold wallet online for a big withdrawal - with the added benefit that, you know, an actual human would check if the transaction is actually valid. It seems to me the solution to this problem (which will keep on happening apparently) is not primarily to make a site that is absolutely penetration proof, but to devise a system where the scope of a breach is guaranteed to be very limited.